To record and store a vast quantity of information as typified by moving image information, advanced optical information media such as read-only optical disks and optical recording disks are required to increase their recording density for increasing the capacity. To meet such a demand, engineers have been engaged in the research and development works targeting a higher recording density.
One such approach relating to digital versatile disks (DVD) is to shorten the wavelength of a recording/reading laser beam and increase the numerical aperture (NA) of a recording/reading optical system objective lens, thereby reducing the spot diameter of the recording/reading laser beam. As compared with CD, DVD is successful in achieving a recording capacity of 6 to 8 folds (typically 4.7 GB/side) by changing the recording/reading wavelength from 780 nm to 650 nm and the NA from 0.45 to 0.6.
Increasing the NA, however, leads to a reduced tilt margin. The tilt margin is a permissible tilt of an optical recording medium relative to an optical system, which depends on the NA. The tilt margin is in proportion toλ/(d·NA3)wherein λ denotes the wavelength of recording/reading beam and “d” denotes the thickness of a transparent substrate the recording/reading beam enters. If the optical recording medium is inclined or tilted relative to the laser beam, a wavefront aberration (or coma) occurs. The coefficient of wavefront aberration is represented by(1/2)·d·{n2·sin θ·cos θ}·NA3/(n2−sin2 θ)−5/2wherein n denotes the refractive index of the substrate and θ is a tilt angle. It is appreciated from these formulae that the tilt margin may be increased and the occurrence of comatic aberration be suppressed by reducing the thickness “d” of the substrate. In fact, the DVD design is such that a tilt margin is secured by reducing the thickness of the substrate to about one half (about 0.6 mm) of the thickness (about 1.2 mm) of the CD substrate.
To record moving images of better quality for a longer period of time, there has been proposed a structure allowing for use of a thinner substrate. In this structure, a substrate of an ordinary thickness is used as a supporting substrate for maintaining rigidity, pits or a recording layer is formed on the surface of the supporting substrate, and a light-transmitting layer of about 0.1 mm thick is formed thereon as a thin substrate. Recording/reading beam reaches the pits or recording layer through the light-transmitting layer. This structure can achieve a higher recording density due to a greater NA because the substrate can be made extremely thin as compared with the prior art. Media having such structure are disclosed in JP-A 10-320859 and 11-120613.
The provision of a light-transmitting layer of approximately 0.1 mm thick allows for use of an objective lens having a large numerical aperture NA, say about 0.85.
Where optical disks are used without being enclosed within cartridges or optical disks can be taken out of cartridges, the light-transmitting layer has the frequent risk of damage and dust deposition on its surface. In the case of optical information media having a thin light-transmitting layer of about 0.1 mm thick, such damage and dust deposition on the surface of the light-transmitting layer have a substantial influence on recording and reading characteristics, as compared with prior art optical information media of the type wherein laser beam is transmitted by a transparent substrate of about 0.6 to 1.2 mm thick.
When optical disks are temporarily stored in their manufacturing process, pin stockers are often used. A plurality of optical disks are vertically stacked on the pin stocker for storage, with the pin inserted into the center holes of the disks. During the pin stocker storage, substantial loads are applied to lower side optical disks so that these disks are brought into intimate contact, possibly leaving contact traces on the disk surface. A common countermeasure is to insert spacers between adjacent disks for preventing the disks from intimate contact. Since contact traces, if imprinted on the surface of the thin light-transmitting layer, will adversely affect recording and reading characteristics, it is important to prevent those disks having a thin light-transmitting layer from mutual intimate contact. The operation of interleaving spacers between adjacent disks, however, is cumbersome and detracts from productivity.